As processes for producing carboxylic acid by oxidizing aldehyde, reactions using potassium permanganate, manganese dioxide or silver oxide as an oxidizing agent are known. However, these processes load a large burden on the environment in view of the generation of highly toxic by-products, the corrosiveness of the oxidizing agents, and the like, so that it is hard to say that they are industrially excellent processes.
On the other hand, oxygen and hydrogen peroxide are inexpensive and non-corrosive, and no by-product is formed or a by-product is harmless water after the reactions, so that an environmental burden is small and thus it can be said that they are excellent oxidizing agents for industrial utilization.
Although processes for forming carboxylic acid from aldehyde using oxygen as an oxidizing agent is known (Non-Patent Document 1), in these processes, an acid, a base, or benzene is used as a solvent in the presence of a heavy metal catalyst such as silver or copper. Moreover, it is also reported that an oxidation reaction of aldehyde with oxygen catalyzed by nickel complex proceeds under mild conditions (Non-Patent Document 2), but it is necessary to use cyclohexanone, acetone, or the like as a solvent.
On the other hand, as processes for producing carboxylic acid from aldehyde using hydrogen peroxide as an oxidizing agent, there are known a process of using an equivalent amount of hexafluoroacetone in a methylene chloride solvent (Non-Patent Document 3) and a process of using formic acid as a solvent (Non-Patent Document 4).
Moreover, as processes for producing carboxylic acid utilizing catalytic oxidation reactions using hydrogen peroxide as an oxidizing agent, there are reported a process of forming a methyl ester using 28% of a sulfuric acid catalyst in a methanol solvent (Non-Patent Document 5), a process of using 5% of a PhSeOOH catalyst in a THF solvent (Non-Patent Document 6), a process of using a catalytic amount (0.9%) of HBr using dioxane as a solvent (Non-Patent Document 7), and a process of using 5% of an SeO2 catalyst in a THF solvent (Non-Patent Document 8). In all these reactions, a water-soluble solvent which forms a homogeneous phase with an aqueous hydrogen peroxide solution, such as formic acid, methanol, THF or dioxane, is used as a solvent.
Thus, in the oxidation reactions of a water-insoluble aldehyde, the use of polar organic solvents as mentioned above is indispensable in order to form a homogeneous solution by dissolving a water-insoluble aldehyde in an aqueous hydrogen peroxide solution. As a result, at the isolation of carboxylic acid which is an objective product, a means for removing the polar organic solvents is necessary and reaction operations and apparatus become complex. Furthermore, the influence and toxicity of the organic solvents themselves on the environment and a human body have been pointed out.    Non-Patent Document 1: Ind. Eng. Chem., 42 768–776 (1950)
Non-Patent Document 2: Chem. Lett., 5–8 (1991)
Non-Patent Document 3: Tetrahedron Lett., 21, 685–688 (1980)
Non-Patent Document 4: Synthesis, 21, 295–297 (1993)
Non-Patent Document 5: J. Org. Chem., 49, 4740–4741 (1984)
Non-Patent Document 6: Tetrahedron Lett., 29, 1967–1970 (1988)
Non-Patent Document 7: Specification of European Patent No. 424,242
Non-Patent Document 8: Synth. Commun., 30, 4425–4434 (2000)